1. Field of the Invention
The present invention relates to information processing apparatus such as image scanners, facsimile machines, copying machines, image sensing apparatus of radiographic image, and so on, photoelectric conversion apparatus used therein, and a driving method thereof.
2. Related Background Art
In the field of linear photoelectric conversion apparatus, active development is recently under way to develop CCD image sensors employing a reduction optical system, and 1:1 type contact image sensors in which a plurality of semiconductor photosensor chips are multiply mounted in a line or in a zigzag pattern. It is common practice to use photodiodes consisting of pn junctions of semiconductors, as light-receiving elements of these photoelectric conversion apparatus.
In the case of amplification type photoelectric conversion apparatus wherein each pn junction part stores a charge obtained by photoelectric conversion, i.e., photo-generated carriers generated by received light and wherein a signal voltage is read out by means of a charge-voltage converter, in order to realize high sensitivity, it is necessary to store the photo-generated carriers effectively and decrease the capacitance of the photoelectric conversion part storing carriers to a value as small as possible.
In the contact image sensors using the light-receiving elements of photodiodes obtained by forming in a semiconductor substrate of a first conduction type areas of an opposite conduction type, however, for example, where the resolution is 300 dpi, the pixel pitch becomes about 84.7 xcexcm, and, in order to extract the photocarriers effectively, it is preferable to form each pn junction in an area approximately equal to that of an aperture portion. However, this results in increasing the capacitance of the pn junction in each photodiode part.
On the other hand, if the area of the pn junction is made smaller in order to decrease the capacitance of the pn junction in the photodiode part, a depletion layer region formed by the pn junction becomes too small relative to the aperture, so as to fail to increase carriers stored in the pn junction part.
In order to solve the above problem, the inventors of the present application proposed the light-receiving element that can have a large light-receiving area while having a small junction capacity, for example, in the specification of European Patent Application Laid-Open No. 1032049.
FIG. 18A is a plan view of a light-receiving element part of the photoelectric conversion apparatus as described in foregoing Application. FIG. 18B is a cross-sectional structure view along a line 18Bxe2x80x9418B of FIG. 18A. FIG. 18C is a diagram showing a potential profile in a direction along the line 18Bxe2x80x9418B of FIG. 18A. FIG. 18D is a diagram showing a potential profile in a direction along a line 18Dxe2x80x9418D of FIG. 18B.
In FIGS. 18A to 18D, reference numeral 1 designates an electrode region, 2 a photoreceptive surface, 3 a p-type semiconductor substrate, 4 an n-type semiconductor region, and 5 a p-type semiconductor region, and the structure is such that light is incident through the photoreceptive surface 2 into a photodiode region having the p-type semiconductor substrate 3, n-type semiconductor region 4, and p-type semiconductor region 5. The electrode region 1 is an n-type high-concentration impurity region and electrode region to which carriers (electrons herein) generated in the photodiode region are gathered to be transferred.
As shown in FIG. 18B, since the conventional photoelectric conversion apparatus is constructed in the structure wherein the p-type semiconductor region 5 of a relatively high concentration is formed on the surface of the p-type semiconductor substrate 3 and wherein the n-type semiconductor region 4 is vertically sandwiched between the surface p-type semiconductor region 5 and the p-type semiconductor substrate 3, the n-type semiconductor region 4 has depletion layers on the both sides, i.e., on the p-type semiconductor region 5 side and on the p-type semiconductor substrate 3 side, thus forming the potential structure as shown in FIG. 18D.
As a result, electrons out of generated electron-hole pairs are gathered in a channel of this potential structure and finally collected into the electrode region 1 with the lowest potential relative to the stored carriers.
However, the prior art sometimes suffers a strong afterimage due to transmission characteristics of photocarriers generated in the light-receiving element. The reason for it is as follows. The carriers generated in the depleted regions in the depth direction diffuse to reach the electrode region with the lowest potential, but rates of the diffusion vary depending upon the potential profile in the depth direction. For this reason, if the potential profile is one that makes the diffusion rates slow, there will appear an afterimage in a next field because of carriers failing to reach the electrode region in certain cases.
Particularly, where the light-receiving element has the photoreceptive surface of a relatively large area, e.g., 30 xcexcmxc3x9730 xcexcm square, or greater than the area equivalent thereto, like the light-receiving elements for the contact image sensors, there can appear an afterimage in a next field because of the carriers failing to reach the electrode region in a period from an end of storage to readout.
The structure in which the flat region with the different potential is provided around the electrode region is disclosed, for example, in European Patent Application Laid-Open No. 1032049, and it improved the afterimage characteristics. However, there are still desires for completely removing the afterimage and for decreasing the afterimage without increase in the number of masks in photolithography.
It is thus an object of the present invention to improve the afterimage characteristics of the light-receiving elements in the photoelectric conversion apparatus.
It is another object of the present invention to improve the afterimage characteristics of the light-receiving elements in the photoelectric conversion apparatus without increase in the number of masks.
It is still another object of the present invention to reduce power consumption in the photoelectric conversion apparatus with excellent afterimage characteristics.
An aspect of the present invention is a photoelectric conversion apparatus comprising:
a light-receiving element having a first semiconductor region of a first conduction type, and a second semiconductor region of a second conduction type for storing a charge generated by photoelectric conversion adjacent to said first semiconductor region;
a readout electrode for reading a signal based on the charge stored in said second semiconductor region; and
at least a pair of electrode portions spaced from each other along a photoreceptive surface so as to place a photoreceptive portion of said second semiconductor region in between, and connected to said first semiconductor region,
wherein a voltage that completely depletes the photoreceptive portion of said second semiconductor region and that can create a potential gradient for moving the charge stored in said second semiconductor region, to the said readout electrode side, is applied to the pair of electrode portions.
It is desirable herein that said first semiconductor region have a semiconductor layer formed so as to be placed between said second semiconductor region and an insulating layer placed above said second semiconductor region and that each of said pair of electrode portions be in contact with said semiconductor layer.
It is also desirable that said semiconductor layer cover the photoreceptive surface of said second semiconductor region.
It is desirable that said second semiconductor region located below said semiconductor layer be completely depleted by depletion layers extending from pn junctions on the upper and lower sides of the second semiconductor region.
It is desirable that said pair of electrode portions be shielded by a shield layer.
It is desirable that an electrode portion placed on the said readout electrode side out of said pair of electrode portions be placed in such a split form as to sandwich said readout electrode along the photoreceptive surface.
It is desirable that said readout electrode be a transfer gate electrode for transferring a charge to a floating diffusion region.
It is desirable that said readout electrode be an anode electrode or a cathode electrode connected to a gate of an amplifying transistor.
It is desirable that immediately before or during a transfer operation to transfer the charge stored in said second semiconductor region or a readout operation to read the signal based on said charge, the voltage that can create said potential gradient be applied during a predetermined period.
It is desirable that during most of an operation period of said photoelectric conversion apparatus, said pair of electrode portions be retained each at an equal potential.
It is desirable that the voltage applied to said first semiconductor region be a reverse bias voltage to said second semiconductor region.
Another aspect of the present invention is a photoelectric conversion apparatus comprising:
a light-receiving element having a first semiconductor region of a first conduction type, and a second semiconductor region of a second conduction type for storing a charge generated by photoelectric conversion adjacent to said first semiconductor region;
a readout electrode for reading a signal based on the charge stored in said second semiconductor region;
at least a pair of electrode portions spaced from each other along a photoreceptive surface so as to place a photoreceptive portion of said second semiconductor region in between, and connected to said first semiconductor region; and
a circuit for retaining said pair of electrode portions at an equal potential during a first period and for applying to said pair of electrode portions a voltage that can create a potential gradient for moving the charge stored in said second semiconductor region, to the said readout electrode side, during a second period.
It is desirable herein that said second period be a period immediately before or during a transfer operation to transfer the charge stored in said second semiconductor region or a readout operation to read the signal based on said charge, through said reading electrode.
It is further desirable that said first period be longer than said second period.
Still another aspect of the present invention is a method of driving a photoelectric conversion apparatus comprising a light-receiving element having a first semiconductor region of a first conduction type, and a second semiconductor region of a second conduction type for storing a charge generated by photoelectric conversion adjacent to said first semiconductor region, and a readout electrode for reading a signal based on the charge stored in said second semiconductor region, said method comprising:
a step of retaining at least a pair of electrode portions spaced from each other along a photoreceptive surface so as to place a photoreceptive portion of said second semiconductor region in between, and connected to said first semiconductor region, at an equal potential during a first period; and
a step of applying to said pair of electrode portions a voltage that can create a potential gradient for moving the charge stored in said second semiconductor region, to the said readout electrode side, during a second period.
In addition, another aspect of the present invention is an information processing apparatus comprising:
the photoelectric conversion apparatus as set forth;
a driving circuit for driving said photoelectric conversion apparatus; and
a signal processing circuit for processing a signal outputted from said photoelectric conversion apparatus.
The foregoing information processing apparatus may be a radiographic image sensing apparatus having a converter for converting a radiographic image into a visible light image.